1. Field of the Invention
The invention relates to a process and a device for reducing the load on a seed crystal during the pulling of a single crystal according to the Czochralski method. The process is used for the production of single crystals from silicon or compound semiconductors for electronics. During the process, a seed crystal which is dipped into a melt is pulled away from the melt with the aid of a pulling device, while a single crystal grows on its underside. During pulling, care is taken that a so-called neck with comparatively small diameter is first produced so that the single crystal can be pulled without dislocations. The single crystal comprises a beginning conical section with increasing diameter, a cylindrical middle section with almost constant diameter, and a conical terminal section with decreasing diameter. Only the cylindrical middle section is of interest as a basic product for the electronics industry.
2. The Prior Art
U.S. Pat. No. 4,973,518 discloses that the neck can become axially loaded with a comparatively heavy weight when the single crystal is being pulled. The application of any transverse force should also be avoided since there is a risk of the neck breaking even under slight loading. To avoid this risk, the conical section is pulled so that it is given a wider diameter at an axial position. This forms a bulge where the single crystal can be supported with a gripping device.
This process has disadvantages for the production of single crystals which have diameters of .gtoreq.200 mm in the cylindrical section. For example, the material needed to produce the bulge cannot be used for electronic purposes. This requires a complicated procedure to produce the special shape to be pulled. Also, the bulge may not successfully be given an exact rotationally symmetric shape. Hence, the possibility of the neck being loaded by transverse forces cannot absolutely be eliminated when the gripping device is engaged. Even if this does not cause the neck to break, there is a risk that the axis of rotation of the single crystal will be shifted. Hence, the cylindrical section will no longer grow in a straight line.